What are some clues a chemical reaction has occurred?

There are several clues to suggest a chemical reaction has taken place. Think about when you mix baking soda and vinegar. You produce a new substance called carbon dioxide that you can see in the form of bubbles. 

Some other signs suggest a chemical reaction could be occurring: 

When iron rusts, it changes colour from grey to reddish-orange.
Anything burning is a chemical reaction with oxygen (called combustion).
An antacid tablet reacts with vinegar to produce carbon dioxide gas.

The new substance (a solid) is not soluble in water.
Food rotting is a chemical reaction that cannot be reversed.

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Chemical reactions may produce some or all of these signs. Some chemical reactions may not show any of these signs. In certain situations, these signs may occur during a physical change. While these clues to chemical changes are useful, they are not always perfect indicators of chemical change.

All chemical reactions have one thing in common: They always produce a new substance. You will be looking at many types of physical and chemical changes to see if you can tell the difference.


  History Connection

Did you know that the Canadian Parliament buildings have roofs coated with copper? The reason the roofs look green and not reddish-brown is that a chemical reaction has occurred. You can see the same effect on the New York’s Statue of Liberty, which is coated with copper.

When copper is exposed to water and oxygen, it undergoes a reaction similar to what hap pens when iron rusts – except that, when copper is oxidized, it changes colour from brown to bluish-green.